Water-soluble polymers are used for various applications and, in particular, as flocculants for processing industrial and urban waste water, dehydration of the sludges generated, as a thickener and as a treating agent for floors. It is well known that aqueous systems of such water-soluble polymers with a high solids content are gelatinous and have very high viscosities, which make them difficult to handle and store. The problem with which a person skilled in the art is faced is to produce such aqueous systems but which have both a high solids content and low viscosity.
Conventional processes for synthesizing these polymers comprise solution, reverse-suspension and reverse-emulsion polymerization. Solution and reverse-suspension polymerization lead to products in powder form which have the drawback of generating dust at the time of use, of dissolving poorly in water and of not being able to form aqueous solutions of polymers in high concentration which can be handled easily. Besides this inconvenient implementation which is intrinsic to the pulverulent form of the product, these two processes are handicapped in terms of production efficiency, on the one hand on account of the low monomer concentration used during polymerization, and on the other hand on account of a drying and/or grinding step leading to an increase in the cycle time and excessive energy consumption costs. As regards the reverse-emulsion polymerization process, which has been known for about two decades, this leads to a product containing a pollutant organic solvent.
To overcome these drawbacks, a novel polymerization technique has been developed leading to aqueous dispersions of water-soluble polymers, the originality of which process lies in the presentation plan, i.e. they are free of pollutant solvent, do not generate any dust, are rapidly water-soluble, are of low viscosity at a high polymer content, and are ready to use. However, this technique requires the development of polymer dispersants which are suitable for the stability of the polymer dispersed in saline or non-saline medium.
Certain authors have prepared cationic or nonionic water-soluble polymers by polymerization of water-soluble monomers in the presence of a polymer dispersant of low mass. EP 170 394 describes a dispersion of polymer gel particles which are greater than 20 .mu.m in size in a solution of poly(sodium acrylate) or poly(DADMAC)--denoting poly(diallyldimethylammonium chloride)--as dispersant. However, this product has the drawback of having a high viscosity after a long period of storage. This viscosity can only be reduced after shear or stirring.
Patents U.S. Pat. No. 183,466, EP 525,751 A1, U.S. Pat. Nos. 4,929,655, 5,006,590 and EP 657,478 A2 propose the case of precipitation-polymerization in saline medium of water-soluble monomers, the polymer of which precipitates in particle form and then disperses by means of stirring and is stabilized by polymer dispersants of low mass, which are themselves soluble in saline medium. The typical dispersants for these polymerizations are poly(DADMAC) or copoly(DADMAC)/(M)ADAMQUAT C16, the latter monomer denoting (meth)acryloxyethyldimethylhexadecylammonium chloride (EP 657,478 A2). In the context of polymerizations in saline medium, patent EP 0,717,056 A2 claims amphoteric water-soluble polymer dispersions based on cationic and anionic monomers (acrylic acid), synthesized in the presence of a dispersant which is a homopolymer of cationic monomers or which is a copolymer of the same cationic monomers with acrylamide.
The typical monomer mixture for this type of polymerization consists of (meth)acrylamide, (meth)acryloxyethyltrimethylammonium chloride and (meth)acryloxyethyldimethylbenzylammonium chloride. The latter monomer plays an important role in the precipitation of the cationic polymer formed during the synthesis and in the formation of particles. U.S. Pat. No. 5,587,415 shows that it is possible to dispense with this monomer by replacing it with another equivalent in which the benzyl group is replaced with a sufficiently hydrophobic C.sub.4 -C.sub.10 alkyl chain. Similarly, U.S. Pat. No. 5,614,602 shows that the same result can be achieved by partially replacing the (meth)acrylamide with an N-alkylacrylamide and/or with an N,N-dialkylacrylamide. The Applicant has now found that stable aqueous dispersions of low viscosity containing a salt, a water-soluble polymer and a dispersant based on cationic polymer containing hydrophobic units can be prepared.